The present invention relates in general to light beam detecting and amplifying circuits and in particular to laser receiver circuits for locating the position of a rotating or scanning laser beam.
Rotating or scanning laser beam transmitters are commonly used in the fields of civil engineering, construction, agriculture, and surveying to establish a reference plane of light useful for taking measurements within a work area. In order to detect the beam, a laser receiver is positioned within the operating range of the laser transmitter and a photosensitive detecting circuit within the receiver is used to monitor the position of the reference plane of light relative to the receiver. Basically, the receiver includes two or more photosensitive detectors that generate photocurrents in response to sensing a strike by the reference plane of light. By measuring the intensity of the beam strike upon each detector, the position of the receiver relative to the reference plane may be determined.
A typical detecting circuit 10 for a laser receiver is illustrated in FIG. 1. A photodetector 12 includes a first detector 14 and a second detector 16. As a laser beam 18 is swept across the photodetector 12, a first detector output current I(A) is generated that is generally proportional to the intensity of the laser beam 18 impinging on the first detector 14. Similarly, a second detector output current I(B) is generated that is generally proportional to the intensity of the laser beam 18 impinging on the second detector 16. The first and second detector output currents I(A) and I(B) generated by the photodetector 12 are typically weak, thus some sort of amplification and/or signal conditioning is required before the signals are useful for determining beam position. Accordingly, a first linear transimpedance amplifier 20 is connected to the first detector 14 to convert the first detector output current I(A) to a first amplifier output voltage A. Similarly, a second linear transimpedance amplifier 22 is connected to the second detector 16 to convert the second detector output current I(B) to a second amplifier output voltage B.
The laser receiver 10 must be able to detect the laser beam 18 over a wide dynamic range of incident beam power to provide a suitable range of operation for practical applications. As a result, the manner in which the receiver 10 determines the beam position must be relatively independent of incident beam power, and thus correspondingly independent of the first and second detector output currents I(A), I(B). Typically, to determine beam position independent of incident beam power, the difference of the first and second amplifier output voltages A, B is computed at the first summer or processing circuit 24. The sum of the first and second amplifier output voltages A, B is computed at the second summer or processing circuit 26, and a processing circuit 28 is provided to compute the beam position by dividing the results of the first summer 24 by the results of the second summer 26. The function of the of processor 28 can be expressed as:             k      1        ⁢                  A        -        B                    A        +        B              ,
where k1 is a constant that reflects an optional gain provided to scale the resulting beam position computation, A is the output voltage of the first linear transimpedance amplifier 20, and B is the output voltage of the second linear transimpedance amplifier.
While the above described technique for determining beam position can provide satisfactory results for determining beam position, there are some practical limitations to the circuit. Because of the necessary wide dynamic range required of the laser receiver 10, the transimpedance amplifiers 20, 22, and the signal detection and processing circuits 24, 26, 28 must be capable of high gain processing, which affects complexity, size, and power consumption of the receiver circuit. Laser receivers are typically batter powered, particularly when employed in applications where conventional electric power lines are not readily available. Thus, battery life is an important factor affecting the utility of the device and the efficiency of those utilizing the device. In view of the above, laser receivers are relatively large in size, contain relatively complex circuitry, and require power in a manner that does not maximize the usefulness of batteries.
The present invention overcomes the disadvantages of previously known receivers by providing circuits that respond logarithmically to a detected light source. The logarithmic responses are processed for locating the position of a target light source such as a rotating or scanning laser beam.
Briefly, a laser receiver circuit includes a photodetector comprising at least one photosensitive device. The photodetector is adapted to produce a first detector output signal and a second detector output signal where the relative amplitudes of the first and second detector output signals are related to the position of a laser beam impinging thereon. A first log signal is determined, which is based upon a logarithmic and optionally scaled calculation utilizing the first detector output signal. Correspondingly, a second log signal is determined, which is based upon a logarithmic and optionally scaled calculation utilizing the second detector output signal. The position of the beam is then computed based upon a subtraction of the second log output signal from the first log output signal.